The reproductive cycle of the scallop Pecten sulcicostatus from the Southern Benguela upwelling system.
Article Type: Report
Subject: Scallops (Physiological aspects)
Scallops (Sexual behavior)
Scallops (Environmental aspects)
Upwelling (Oceanography) (Influence)
Authors: Arendse, Dale C.Z.
Andersen, Sissel
Blake, Norman J.
Pitcher, Grant C.
Pub Date: 04/01/2008
Publication: Name: Journal of Shellfish Research Publisher: National Shellfisheries Association, Inc. Audience: Academic Format: Magazine/Journal Subject: Biological sciences; Zoology and wildlife conservation Copyright: COPYRIGHT 2008 National Shellfisheries Association, Inc. ISSN: 0730-8000
Issue: Date: April, 2008 Source Volume: 27 Source Issue: 2
Topic: Event Code: 310 Science & research
Product: Product Code: 0913070 Scallops NAICS Code: 114112 Shellfish Fishing SIC Code: 0913 Shellfish
Geographic: Geographic Scope: South Africa Geographic Code: 6SOUT South Africa
Accession Number: 179387568
Full Text: ABSTRACT The reproductive cycle of Pecten sulcicostatus is described as part of an investigation into the potential commercial culture of this species in South Africa. Scallops were collected monthly from False Bay, from August 2004 to October 2005, to determine seasonal variation in the gonadosomatic index (GSI) and to assess associated histological changes within the gonads. The reproductive cycle of P. sulcicostatus demonstrated clear seasonality. The mean GSI was highest from June to September (winter to early spring), and lowest from November to January (late spring to summer). The GSI findings were corroborated by histological analysis of the gonads with the mean oocyte diameter reaching a maximum in August and a minimum in November. Seasonal stratification and corresponding changes in phytoplankton biomass are considered to control the reproductive cycle of P. sulcicostatus in False Bay as winter spawning and the subsequent decline in the GSI coincide with the transition to spring upwelling conditions and a decline in bottom water temperature and food availability.

KEY WORDS: scallop, reproduction, gonadosomatic index, upwelling, Pecten sulcicostatus

INTRODUCTION

The scallop Pecten sulcicostatus (Sowerby 1842) is endemic to the inner continental shelf of South Africa (Dijkstra & Kilburn 2001) and is distributed on the southwest and south coast, from False Bay to East London (Fig. 1). Pecten sulcicostatus is typically found at sublittoral depths between 20 and 80 m (De Villiers 1976), is free-living on sand or muddy sand and may reach a maximum size of 106 mm (Branch et al. 1994).

De Villiers (1976) established the densities of populations of P. sulcicostatus in False Bay and Mossel Bay (Fig. 1) and also estimated their rates of growth. Scallops in False Bay were unevenly distributed with densities ranging from 1-106 scallops per 10-min trawl with the highest densities near the center of the bay. The density of scallops in Mossel Bay was found to be lower, ranging from 1-4 scallops per 10-min trawl. Estimates of 4-5 y to reach marketable size (90 mm) were based on shell surface structure, but no recommendation as to the suitability of this resource as a potential fishery was made because of the absence of information on recruitment and mortality. De Villiers (1976) did, however, suggest that the farming potential of this scallop be investigated because the meat obtained an adequate size for marketing.

An understanding of reproductive processes is central to the management of any commercial fishery (Barber & Blake 2006), because information is provided on the recruitment and population dynamics of the species on which the fishery is based (Williams & Babcock 2004). Understanding the reproductive cycle of a species is also a requirement for the successful cultivation of a species, providing information useful to the collection, conditioning, and spawning of broodstock. The reproductive cycle of many scallop species has been investigated over the past few decades (Sastry 1979, Barber & Blake 1983, MacDonald & Bourne 1987, Strand & Nylund 1991, Narvarte & Kroeck 2002) and in most cases distinct peaks in spawning activity have been demonstrated. These peaks may differ temporally in association with the latitude of the population (Barber & Blake 1983, Strand & Nylund 1991) and in most temperate species the reproductive cycle has been linked to water temperature and food availability (Barber & Blake 2006).

Pecten sulcicostatus has been poorly studied and its biology and life history are unknown. This study undertook to describe the reproductive life cycle of the P. sulcicostatus population in False Bay, thereby providing knowledge fundamental to the future cultivation of this species.

MATERIALS AND METHODS

False Bay is the largest bay in South Africa with an area of approximately 900 [km.sup.2]. Scallops were collected off Millers Point by scuba divers at a depth of 20-22 m (Fig. 1). Seawater temperature was recorded to the nearest 0.5[degrees]C at the depth of collection using a Santo D3 divers watch.

Scallops were collected monthly from August 2004 to October 2005 and sample size varied from 13 to 46 individuals (Table 1). The scallops were transported to the laboratory in moist paper towels in Styrofoam boxes at 14-16[degrees]C and processed on the same day. The shell height was measured to the nearest 1 mm using a digital caliper and for the purpose of analysis scallops were divided into two size ranges: those <70 mm in shell height, and those [greater than or equal to] 70 mm (Table 1). The whole scallop, shell, soft tissue, gonad, and abductor muscle were individually weighed (wet weight, ww) using a Denver Instrument APX-602 scale. The gonadosomatic index (GSI) was calculated as follows (MacDonald & Bourne 1987):

GSI = [Gonad weight (ww)/Somatic tissue (ww)] x 100

Gonads were prepared for histological studies by placing them individually in Dietrich fixative (Yevich & Barszcz 1977) for one hour. To further expose the gonads to the preservative they were removed from the fixative, cut into sagittal sections, wrapped in cheesecloth, and returned to the fixative for 24 h to ensure preservation of the interior tissues. The sections were washed in seawater and stored in 70% ethanol. Further processing included six changes of tissue dehydrant and three changes of clearing agent before the tissues were embedded in Paraplast, following the procedures of Barber & Blake (1983). Tissues were sectioned (5-7 [micro]m thick), placed on glass slides, stained with Harris' hematoxylin-eosin, and covered with a glass cover slip (Yevich & Barszcz 1977).

[FIGURE 1 OMITTED]

Oocyte areas were established for a selected number of scallops (Table 1) on each sample date, using a Zeiss Photomicroscope and Boeckeler video imaging system. Fifty oocytes were measured from each gonad. Oocytes prior to maturation were targeted and included only those in which the nucleolus was clearly visible in the nucleus, thereby indicating a higher probability of sectioning through the center of the oocyte. Oocyte areas rather than diameters were measured, because they are less influenced by sectioning techniques, and therefore provide more reliable data from which the oocyte diameter may be calculated (Dukeman et al. 2005).

Statistical analyses were completed using Statistica version 6.1 (Statsoft Inc.) and Sigma Stat 3.1 (Systat Software Inc.). The Shapiro-Wilk test was used to test for normality, and the Levene Test was used to test for equal variance. A t-test or a Mann-Whitney U-test was used to test for differences in the GSI between the two size groups (<70 mm; [greater than or equal to] 70 mm) of scallops collected each month. The monthly mean GSI was established by pooling these two size groups and the Kruskal-Wallis one way ANOVA by Ranks test was used to compare the GSI between months. The Dunn Post hoc Test was used to determine which months were different. The ANOVA test was used to test for differences in the mean oocyte diameters between months and the Tukey HSD Test was used to establish which months were different. The significance level ([alpha]) was set at 0.05.

RESULTS

Water temperature at the time of sample collection at 20-22 m depth ranged from 13.0-15.0[degrees]C (Fig. 2A). Winter temperatures (June to August) tended to be higher than those temperatures during the remainder of the year. Temperatures were lowest during late spring and summer (November to January).

Pecten sulcicostatus was found to be a functional hermaphrodite with the gonads comprising both whitish testes and orange ovaries. The shell height of the scallops collected during the study ranged from 39-110 mm. Only during December 2005 was there a significant difference (P = 0.002) in the GSI between the two size groups (<70 mm; [greater than or equal to] 70 mm) of scallops. During this month the mean GSI for the scallops <70 mm was 7.2% ([+ or -] 1.8 sd, n = 22) and for scallops [greater than or equal to] 70 mm the mean GSI was 5.2% ([+ or -] 1.6 sd, n = 20). Consequently the two size groups were pooled for further analysis.

[FIGURE 2 OMITTED]

A clear seasonal pattern was evident in the GSI, which was lowest from November to February and highest from June to September (Fig. 2B). The minimum mean GSI value of 6.2% ([+ or -] 1.9 sd, n = 42) was recorded in December 2004 and the maximum of 14.4% ([+ or -] 4.0 sd, n = 24) in August 2005. As evident from the standard deviation, there was significant variation in the GSI between individual scallops within each month, particularly from May to August 2005 (Fig. 2B).

The frequency of each reproductive phase as determined by four groupings of the GSI (Fig. 3) showed gonads in a vegetative stage (i.e., with the lowest GSI values of 4% to 8%) to be present year-round, but most common from October to February (and May). Gonads undergoing vitellogenesis (i.e., with the highest GSI values of 18% to 22%) were present only from July to September (Fig. 3).

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Monthly mean oocyte diameter demonstrated a similar seasonal pattern to that of the GSI (Fig. 4) with a minimum mean diameter of 36.6 [micro]m ([+ or -] 3 sd, n = 13) in November 2004 and a maximum of 54.7 [micro]m ([+ or -] 2.7 sd, n = 7) in August 2005. There was a significant difference between the minimum and maximum values.

[FIGURE 5 OMITTED]

Gonad histology showed that from November to December when the mean GSI was low, more than half of the scallops had either spawned and/or the oocytes were undergoing resorption (Fig. 5A). From January to March most oocytes were in the early-maturation stage (Fig. 5B), and during this period no resorption of oocytes was observed in the follicles. From April to June most oocytes were in the mid-maturation stage (Fig. 5C). From June to August most oocytes were ripe and during this period few amoebocytes were present providing further evidence of gonads in the process of maturation (Fig. 5D). At this time the follicle membrane was very thin and oocyte nuclei were not visible. New and developing oocytes were present during all months.

DISCUSSION

The reproductive cycle of P. sulcicostatus in False Bay demonstrated a clear seasonal cycle. The GSI, supported by histological analysis, showed spawning to occur predominantly between June and September, peaking in August and September, after which there was a sharp decrease in the mean GSI. However, minor spawning events throughout the year cannot be excluded, as oocytes appear to develop continuously throughout the reproductive cycle. This trend in spawning period is similar to that of the Australian scallop P. fumatus, which also exhibits peak spawning activity in winter and early spring (Young et al. 1999). However, most scallop species spawn primarily in summer after an increase in water temperature (Beninger 1987, Shafee 1980, Strand & Nylund 1991, Roman et al. 2002, Williams & Babcock 2005).

The maximum GSI for P. sulcicostatus (14.4%) was similar to that reported for Argopecten. irradians (14.5%; Barber & Blake 1983), but considerably lower than that reported for Aequipecten tehuelchus (25%; Narvarte & Kroeck, 2002), Pecten maximus (38.8%; Strand & Nylund 1991), Patinopecten caurinus (21%; MacDonald & Bourne 1987) and Aequipecten irradians (22%; Sastry 1979). A higher mean GSI may be a function of a higher degree of synchronicity in spawning within populations of these species.

Recognizable stages in the reproductive cycle of a population typically include activation, gamete growth, ripening of gametes, spawning, and an inactive or "resting" period (Sastry 1979). In this study the low frequency of totally spent gonads demonstrates the absence of a postspawning resting period in the False Bay population. Instead, gametes begin developing immediately after spawning and newly developing oocytes are present all year. Considerable variability in the GSI and oocyte diameter between individuals within each month also implies a low level of spawning synchronicity in P. sulcicostatus.

Water temperature and food availability are the parameters most often cited as controlling the reproductive cycle of scallops (Sastry 1979, Barber & Blake 1981, Cruz & Villalobos 1993, Wada et al. 1995). For A. irradians, P. fumatus, and P. maximus the parameter most important in triggering spawning is temperature. However, these species are typically subjected to large temperature differences between winter and summer ranging from 6[degrees]C to 13[degrees]C (Barber & Blake 1983, Strand & Nylund 1991, Young et al. 1999).

In contrast, bottom temperatures in False Bay vary within a small range. False Bay forms an integral part of the southern Benguela upwelling regimen and demonstrates strong seasonality in water column stratification (Shannon 1985). Little or no stratification is evident in winter but the system becomes strongly stratified in summer owing to solar heating of the surface waters and the inflow of cold bottom water into the Bay after upwelling on the adjacent shelf (Eagle & Orren 1985, Swart & Largier 1987, Boyd et al. 1985). Consequently, surface temperatures decrease by approximately 5[degrees]C in winter and bottom temperatures decrease by 1[degrees]C to 3[degrees]C in summer (Atkins 1970). These observations are consistent with the temperatures recorded during the 14 mo of this study. Winter spawning and a decline in the GSI seem therefore to coincide with the transition to spring upwelling conditions and a decline in bottom temperatures. However, it remains questionable as to whether this relatively small decline in temperature alone is able to control the reproductive cycle of P. sulcicostatus in False Bay.

Coincident with seasonal stratification are variations in phytoplankton biomass and the consequent availability of food for scallops. False Bay is an area of high phytoplankton biomass, which tends to peak in late summer when the Bay is strongly stratified (Hutchings et al. 2006, Pitcher & Weeks 2006). Food availability to scallops is likely to be greatest after the decay of these late summer blooms and their sedimentation to the benthos further enhanced by winter mixing. It is at this time that the GSI is highest. The intrusion of cold food-depleted bottom waters into the Bay after the transition to spring upwelling conditions coincides with a decline in the GSI. It thus seems likely that temperature together with food availability may control the reproductive cycle of P. sulcicostatus in False Bay. The high phytoplankton biomass throughout the year may nevertheless explain the low frequency of spent individuals, and the presence of newly developing oocytes year round.

The results of this investigation indicate that broodstock should not be collected for conditioning and spawning purposes between October and December because the GSI decreases at this time and few mature oocytes are present. Further research towards successful cultivation of P. sulcicostatus should seek to establish the environmental parameters important in the maturation of the gonads and those that trigger spawning.

ACKNOWLEDGMENTS

The authors thank Gottlieb and Aldo Scheun (BS Divers) for collecting the scallops. The project was partly funded by the Norwegian Agency for Development Co-operation (Grant Number: 3004).

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DALE C. Z. ARENDSE, (1,4)* SISSEL ANDERSEN, (2) NORMAN J. BLAKE (3) AND GRANT C. PITCHER (1)

(1) Marine and Coastal Management, Private Bag X2, Rogge Bay, 8012, Cape Town, South Africa; (2) Institute of Marine Research-Austevoll, N-5392 Storebo, Norway; (3) College of Marine Science, University of South Florida, St. Petersburg, Florida, 33701; (4) Department of Zoology, University of Cape Town, 7701, South Africa

* Corresponding author. E-mail: dalearendse@deat.gov.za
TABLE 1.
Summary of sampling dates, shell height, number of
scallops <70 and [greater than or equal to] 70 mm
and the sample size for histological analysis.

                                Larger
                                Group
                                  N
                  Shell      ([less than
Date of           Height     or equal to]
Collection      Range (mm)      70 mm)

23 Aug. 2004      45-110          10
20 Sept. 2004     46-102          25
18 Oct. 2004      49-100          21
19 Nov. 2004      54-107          23
20 Dec. 2004      53-103          20
20 Jan. 2005      52-102          15
22 Feb. 2005      44-101          20
16 Mar. 2005      59-99           20
13 Apr. 2005      47-97           19
19 May 2005       39-101          20
23 Jun. 2005      52-79           14
18 Jul. 2005      43-102          10
15 Aug. 2005      46-95           20
27 Sept. 2005     45-84           17
18 Oct. 2005      81-98           13

                 Smaller
                  Group
Date of             N         Histology
Collection       (<70 mm)         N

23 Aug. 2004        24            0
20 Sept. 2004       21            0
18 Oct. 2004        21            0
19 Nov. 2004        23            13
20 Dec. 2004        22            6
20 Jan. 2005        25            2
22 Feb. 2005        20            0
16 Mar. 2005        2             5
13 Apr. 2005        8             7
19 May 2005         7             8
23 Jun. 2005        12            4
18 Jul. 2005        18            6
15 Aug. 2005        4             7
27 Sept. 2005       8             3
18 Oct. 2005        0             7
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